Defect healing of deposited titanium alloys

ABSTRACT

A method for treating a deposited titanium-base material from an initial condition to a treated condition includes a rapid heating and a rapid cooling. The heating is from a first temperature to a second temperature, the first temperature being below a β transus and the second temperature being below above the β transus. The cooling is from the second temperature to a third temperature below an equilibrium β transus.

BACKGROUND OF THE INVENTION

The invention relates to deposition of Ti-based materials. Moreparticularly, the invention relates to addressing deposition defects.

A growing art exists regarding the deposition of Ti-based materials. Forexample, electron beam physical vapor deposition (EBPVD) may be used tobuild a coating or structural condensate of a Ti alloy atop a substrateof like or dissimilar nominal composition. Such techniques may be usedin the aerospace industry for the repair or remanufacture of damaged orworn parts such as gas turbine engine components (e.g., blades, vanes,seals, and the like).

Deposition defects, however, potentially compromise the condensateintegrity. One group of such defects arises when a droplet of materialis spattered onto the substrate or the accumulating condensate. The meltpool may contain additives not intended to vaporize and accumulate inthe condensate. For example, U.S. Pat. No. 5,474,809 discloses use ofrefractory elements in the melt pool. Once the droplet lands on thesurface (of the substrate or the accumulating condensate) furtherdeposition builds atop the droplet and the adjacent surface. Along thesides of the droplet, there may be microstructural discontinuities inthe accumulating material due to the relative orientation of the sidesof the droplet. As further material accumulates, these discontinuitiesmay continue to build all the way to the final condensate surface.

SUMMARY OF THE INVENTION

One aspect of the invention involves a method for treating a depositedtitanium-base material from an initial condition to a treated condition.The material is heated from a first temperature to a second temperature.The first temperature is below an equilibrium β transus. The secondtemperature is above the equilibrium β transus. The heating includes aportion at a rate in excess of 5° C./s. The material is cooled from thesecond temperature to a third temperature below the equilibrium βtransus.

In various implementations, the heating may be to a peak at least 10° C.above a non-equilibrium β transus. The material may be above theequilibrium β transus for a brief period (e.g., no more than 2.0seconds). The heating and cooling may have sufficient rates to maintaina characteristic grain size of at least a matrix of the material below100 μm. In the initial condition, the material may include a number ofdefects having trunks with microstructures distinct from amicrostructure of a matrix of the material. In the treated condition,the trunks' microstructures may be essentially integrated with thematrix microstructure.

In one group of implementations, the heating may be to a peak 10-50° C.above a non-equilibrium β transus. The material may be above theequilibrium β transus for a very brief period (e.g., no more than 1.0seconds). In another group of potentially overlapping implementations,the heating may be to 1-30° C. above the equilibrium β transus for asomewhat longer period (e.g., 1.0-5.0 seconds). The cooling may besufficiently rapid to limit β growth to a characteristic size smallerthan 100 μm. One group of materials consist in largest weight parts oftitanium, aluminum, and vanadium. An exemplary material thickness may beat least 2.0 mm (e.g., for relatively thick structural and repairmaterial, contrasted with thinner coatings).

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical micrograph of a Ti-6Al-4V condensate atop a likesubstrate and showing defects.

FIG. 2 is a view of a healed condensate.

FIG. 3 is a temperature-time diagram showing healing processes.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 shows a condensate 20 accumulated atop a surface 22 of asubstrate 24. Exemplary condensate thickness may be from less than 0.2μmm (e.g., for thin coatings) to in excess of 2 mm (at leastlocally—e.g., for structural condensates such as certain repairs). Thecondensate has a first defect 26 triggered by a spattered molybdenumdroplet 28 that landed atop the surface 22. Exemplary droplet sizes are30-500 μm (measured as a characteristic (mean/median/mode) transversedimension). The defect comprises a trunk 30 extending from the droplet28 toward the condensate surface (not shown). A second defect 32 isshown and may have been triggered by a droplet below the cut surface ofthe view.

The exemplary deposition is of nominal Ti-6Al-4V condensate atop a likesubstrate. Alternate depositions may include Ti-6Al-2Sn-4Zr-2Mo andTi-8Al-1Mo-1V. The deposition may be from a melted ingot at leastpartially through a pool containing one or more refractory or otherelements which may be essentially non-consumed during deposition (e.g.,a pool formed from a 30%Mo-70%Zr mixture). Accordingly, the droplets maytend to have composition similar to the surface layers of the pool. Inthe absence of the non-consumed pool additive, the droplet 28 might havea similar composition to the ingot yet still produce similar defects.Many droplets in systems using an Mo-containing pool would have Moconcentrations of at least 10% by weight; others at least 20%. This maybe somewhat less than the Mo percentage of the non-expending poolmaterial to reflect possible dilution by deposition material elements inthe pool.

In the exemplary implementation, the substrate 28 has an α-βmicrostructure of medium to coarse grains (e.g., 10-40 μm characteristicgrain size (e.g., mean) or about ASTM 10.5-6.5). An exemplary 10-20% byweight of the substrate is β phase with the remainder essentially aphase. The condensate matrix (away from the defects) also has an α-βmicrostructure but of very fine grains (e.g., acicular α grains of 5-10m in length and 2-5 μm in thickness, lengthwise oriented along thecondensate growth/deposition direction). The trunk size will depend, insubstantial part, upon the droplet size. Exemplary trunk diameters arefrom about 20 μm to about 50 μm. However, much larger trunks arepossible. The trunks have a columnar α-β microstructure. Thismicrostructure may have a characteristic grain size several timesgreater than that in the matrix and the grains may be elongated in thedirection of accumulation (i.e., away from the substrate). Particularlyin the case of very large diameter trunks (e.g., in excess of 100 μm indiameter), there may be porosity around the trunk. The graindiscontinuity at the trunk-matrix interface and the particular alignmentof trunk grains may cause structural weaknesses affecting, inter alia,ductility, fracture toughness, fatigue resistance, fretting fatigueresistance, corrosion resistance, wear resistance, crack nucleationresistance, and the like.

We have, accordingly, developed heat treatment regimes for healingworkpieces having such defects. FIG. 2 shows a condensate 50 afterhealing. In this view, the substrate is not shown. The condensatesurface 52 is, however, shown. A defect within the condensate had beencaused by a droplet 54. The resultant trunk had propagated all the wayto the surface forming a bulge 56. The healed condensate, however, showsessentially no remaining artifacts of the defect other than the originaldroplet 54 and bulge 56. The trunk has been microstructurally integratedwith the condensate matrix as a fine β equiaxed microstructure (e.g.,grain size of 10-100 μm (˜ASTM 10.5-3.5)). Additionally, laminarvariations in chemistry or microstructure may be diminished oreliminated, thereby increasing the isotropy of the condensate mechanicalproperties. The bulge 56 may be removed (e.g., during a subsequentsurface machining). The healing permits the workpiece (e.g., a blade,vane, seal, or the like) to be operated at ambient temperature (e.g.,0-40° C.) and/or at elevated temperatures (e.g., above 250° C., such asin the range of 300-500° C.) essentially without increased chances offailure.

EXAMPLE 1

In a first exemplary healing method, the workpiece is initially at roomtemperature (condition/location 100) on the temperature against timeplot of FIG. 3. FIG. 3 further shows a line 102 representing theequilibrium β transus (T^(e) _(β)) as well as curves 104 and 106 ofnon-equilibrium β transus temperatures for the condensate and substrate,respectively (specific to the microstructural and thermal history ofthis Example 1). The equilibrium β transus temperature is a function ofchemistry only. Because the exemplary condensate and substrate have thesame chemistry, they share the same equilibrium β transus temperature(e.g., 960-1010° C. for Ti-6Al-4V). The non-equilibrium β transustemperature is a function of composition, grain size/morphology, andheating rate. The smaller the grains, the lower the transus temperature.The more rapid the heating rate, the higher the transus temperature. Atvery slow heating rates, the equilibrium transus temperature equals thenon-equilibrium transus temperature. Given very fine condensate grains,medium to coarse substrate grains, and a heating rate of approximately100° C./sec, it is estimated that the difference between thenon-equilibrium β transus of the two structures is about 100° C.

In a first stage, the workpiece is heated 109 moderately above thecondensate non-equilibrium β transus (condition/location 110). Thisheating may be under vacuum or in an inert atmosphere. This heating isadvantageously rapid (e.g., occurring at a rate of 5-100° C./s orgreater) so as to prevent excessive grain growth. Excessive grain growth(e.g., above 150 μm (˜ASTM 2.5) or even 100 μm (˜ASTM 3.5), dependingupon the application) is disadvantageous because it excessively reducesstructural properties including one or more of ductility, fracturetoughness, fatigue resistance, fretting fatigue resistance, corrosionresistance, wear resistance, crack nucleation resistance, and the like.The heating reaches a peak of ΔT₁ above the condensate non-equilibrium βtransus of approximately 10-50° C. The heating substantially convertsthe condensate microstructure to β. The upper limit on ΔT₁ will reflectthe microstructural/thermal history and is advantageously sufficientlylow to avoid excessive β grain growth (in view of time considerationsdiscussed below). The lower limit is advantageously high enough toprovide essentially complete transition to the β phase (α+β to β). Thesubstrate may be essentially unaffected.

In a second stage, the workpiece is rapidly cooled 111 back to roomtemperature (condition/location 112). This heating may be under the samevacuum or atmosphere as the first stage. During this cooling, metastablemartensite may accumulate in the condensate and the substrate. Thecooling is advantageously sufficiently rapid to further limit β (graingrowth. The rapid heating and cooling maintain the condensate above itsequilibrium β transus for a time interval sufficiently brief to avoidthe excessive β grain growth noted above while providing the β phasetransition. An exemplary time interval above the equilibrium β transusis 1.0 seconds or less. Once below the β transus, the β will transformto α+β in a β-transformed (also known as transformed β) microstructure.This microstructure is characterized by preservation of the boundariesof the prior β grains with a (e.g., in needle- or platelet-like form) ina β matrix within such boundaries. The rapid heating and cooling mayprovide a sufficiently short time at elevated temperature (in view ofthe magnitude of such temperature) to greatly limit oxidation even ifthe procedure is performed in an ambient atmosphere rather than undervacuum or an inert atmosphere. Thus, especially for workpieces to beexposed to relatively low thermal and mechanical stresses (e.g., somenon-rotating turbine engine parts), there may be greater environmentalflexibility during the heating and/or cooling.

An optional third stage (not shown in FIG. 3) involves annealing/agingby heating the workpiece to an annealing temperature (e.g., in thevicinity of 500-600° C. for the exemplary Ti alloy). The exemplaryannealing/aging is for a period of 1-24 hours and is effective toeliminate the martensite but without producing β grain growth. This mayleave the condensate as essentially fine grain β (e.g., broadly smallerthan 150 μm, more narrowly smaller than 100 μm, and preferably smallerthan 50 μm (˜ASTM 2.5, 3.5, and 5.5, respectively)) plus the dropletsand leaves the substrate as essentially medium to coarse grain β-β.However, the cooling of the second stage may be sufficiently slow toavoid martensite formation, in which case it is particularly appropriateto omit the aging/annealing.

EXAMPLE 2

In a second exemplary healing method, the initial heating stage 119 issimilarly rapid but to a temperature above the equilibrium β transus butbelow the condensate non-equilibrium β transus. The resultingcondition/location 120 may be at a temperature of ΔT₂ above theequilibrium β transus (e.g., by 10-30° C.). The upper limit on thisrange is advantageously effective to avoid excessive β grain growth. Thelower limit on this range is advantageously high enough to provideessentially complete transition to the β phase (α+β to β). The substratemay be essentially unaffected.

Rather than being immediately followed by rapid cooling, the workpieceis maintained 121 at such a temperature for a moderate time interval(e.g., of about two seconds, more broadly 1.5-4.0 seconds or 1.0-5.0seconds) to achieve a condition/location 122. It is during this timeinterval that the condensate microstructure changes to fine β as incondition/location 110. Thereafter, a rapid cooling 123 may transitionthe workpiece to a condition/location 124 similar to 112 and may, inturn, be followed by a similar annealing/aging if appropriate ordesired.

The heating and cooling may be performed by a variety of techniques. Onefamily of rapid heating techniques provides highly local heating(exemplary such techniques include induction heating, laser heating,electron beam heating, and the like). Such heating facilitates heatingof the condensate with less substantial heating of the substrate,thereby minimizing any structural effects on the substrate. The desiredheating depth may be controlled in view of the coating thickness bymeans including frequency control of induction heating, beam intensityof laser or electron beam heating, and the like. With someimplementations of such heating, the heating may progress across thecondensate (e.g., by moving or reorienting the workpiece relative to theheating source). Direct electrical resistance heating may be used tomore generally heat the workpiece. The exemplary rapid cooling may beperformed by forced cooling with an inert gas (especially when theheating is performed under vacuum), forced air cooling, liquid quench(e.g., in oil or water), and the like.

One or more embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, details of the chemical composition of the particular substrateand condensate and of the physical configuration of the substrate andthickness of the condensate may influence details of any particularimplementation. Accordingly, other embodiments are within the scope ofthe following claims.

1. A method for treating a deposited titanium-base material from aninitial condition to a treated condition comprising: heating from afirst temperature to a second temperature, the first temperature beingbelow an equilibrium β transus, the second temperature being above theequilibrium β transus, and the heating including a portion at a rate inexcess of 5° C./s; and cooling from the second temperature to a thirdtemperature below the equilibrium β transus.
 2. The method of claim 1wherein: the heating is to a peak at least 10° C. above anon-equilibrium β transus; and the material is above the equilibrium βtransus for a period of no more than 2.0 seconds.
 3. The method of claim1 wherein: the heating and cooling have sufficient rates to maintain acharacteristic grain size of at least a matrix of the material smallerthan 100 μm.
 4. The method of claim 1 wherein: in the initial condition,the material includes a plurality of defects having trunks with amicrostructure distinct from a microstructure of a matrix of thematerial; and in the treated condition, the trunks' microstructure havebeen essentially integrated with the matrix microstructure.
 5. Themethod of claim 1 wherein: the heating is to a peak 10-50° C. above anon-equilibrium β transus; and the material is above the equilibrium βtransus for a period of no more than 1.0 seconds.
 6. The method of claim1 wherein: the heating is 1-30° C. above the equilibrium β transus for aperiod of 1.0-5.0 seconds; and the cooling is sufficiently rapid tolimit β growth to a characteristic size smaller than 100μm.
 7. Themethod of claim 1 wherein: the material consists in largest weight partsof titanium, aluminum, and vanadium; and the material has a maximumthickness of at least 2.0 mm.
 8. The method of claim 1 wherein: thematerial is on a titanium-base substrate; and the heating leavesessentially unaffected a microstructure of a major portion of thesubstrate.
 9. The method of claim 1 wherein: the heating is selectedfrom the group consisting of direct resistance heating, inductionheating, electron beam heating, and combinations thereof.
 10. The methodof claim 1 further comprising: depositing the material on atitanium-base substrate.
 11. The method of claim 10 wherein: thedepositing comprises electron beam physical vapor deposition.
 12. Themethod of claim 10 wherein: the material and the substrate consistessentially of an alloy of 5-7 weight percent aluminum, 3-5 weightpercent vanadium, balance titanium, with less than 3 weight percentother components.
 13. The method of claim 1 further comprising:maintaining the material at a temperature of 500-660° C.
 14. The methodof claim 1 further comprising: an annealing and aging step.
 15. Themethod of claim 1 used to repair a gas turbine engine component having atitanium-base substrate.
 16. The method of claim 15 further comprising:operating the repaired component at a temperature in excess of 250° C.17. The method of claim 1 wherein in the treated condition a laminarvariation in at least a first alloy component is less than in theinitial condition.
 18. A method for treating a deposited titanium-basematerial, the material initially having: a matrix having first nominalchemistry and a first characteristic grain size and first characteristicgrain structure; a plurality of spits within the matrix and having: adroplet having a higher level of refractory impurities than the matrix;and a trunk extending from the droplet and having essentially the samechemistry as the matrix, but a larger second characteristic grain sizeand less equiaxed second grain structure, the method comprising: heatingthe material; and cooling the material, the heating and cooling beingsufficiently rapid to convert an α-β microstructure of the material toan essentially β-transformed microstructure, optionally includingmetastable martensite, and having a characteristic grain size smallerthan 100 μm.
 19. The method of claim 18 wherein: the first nominalchemistry is essentially Ti-6Al-4V; the second nominal chemistry isessentially Ti-6Al-4V; and the material is atop an essentially Ti-6Al-4Vsubstrate.
 20. The method of claim 18 wherein: at least some of thespits are further characterized by porosity adjacent their trunks; andat least some of the porosity is healed.
 21. The method of claim 18wherein: the droplets comprise at least 10% of one or a combination ofrefractory metals, by weight.
 22. The method of claim 18 furthercomprising: an annealing/aging step effective to essentially. eliminatethe martensite.
 23. The method of claim 18 wherein: the material is arepair material on a turbine engine component.
 24. A component having: aTi-based metallic substrate; and a Ti-based condensate atop thesubstrate and having: a surface; a plurality of embedded droplets belowthe surface; regions directly between the droplets and the surfacecharacterized by an essentially β-transformed microstructure of acharacteristic grain size below 100 μm.
 25. The component of claim 24wherein: at least some of said droplets are at least 200 μm below thesurface.
 26. The component of claim 24 wherein: at least some of saiddroplets are at least 20 μm in characteristic transverse dimension. 27.The component of claim 24 wherein: at least some of said dropletscomprise at least 20% Mo, by weight.
 28. The component of claim 24wherein: the substrate and the condensate each consist essentially ofTi-6Al-4V.
 29. The component of claim 24 being one of a gas turbineengine compressor blade, fan blade, disk, drum rotor, bearing housing,vane, and seal element.